I believe that they require damp conditions to feed on earthworms which explains why you see them occasionally on roadside verges at dawn and the dusk flights from cover to open fields in the winter. This would suggest drying out would be a problem.
The Migration Atlas suggests that the British & Irish wintering population splits - 37% Russia & Latvia, 25% Finland, 14% Britain, 12% Sweden and 10% Norway so even during periods when the continental winterers had arrived, this would suggest one in seven shot was a local breeder.
If so, does logic dictate that the percentage local population shot each year might be (national number shot * 0.14)/(local winterers)?
Playing with numbers - worst case 200,000 shot and 100,000 local winterers (28%) and best case 100,000 shot and 180,000 local winterers (8%).
As always, the primary driver of declines is destruction of habitat in southern England.
All the best